1. Field of the Invention
This invention relates generally to semiconductor power amplifiers and more particularly to Class C bipolar transistor amplifiers having limited output power under overdrive conditions.
2. Description of Related Art
Class C bipolar transistor amplifiers are generally well known and have been widely used in solid state transmitters, particularly transmitters used in connection with radar systems. Along with improvements being made in such apparatus, improvements have also been made in bipolar power transistor technology, such as with the introduction of polysilicon emitters and silicon germanium (SiGe) bases to extend their performance limits. Such improvements have resulted in increasing the power and frequency limits of these devices. However, in doing so, some of the unwanted parasitics which hinder power performance have been removed, thus expanding the limit of maximum power which can be generated. Nevertheless, a new requirement has been presented to transistor designers, that being the fabrication of transistors that saturate at a particular power output level. A second added requirement has been to pass a voltage standing wave ratio (VSWR) survival test during an overdrive condition. These requirements have resulted in a significant amount of failed parts and a considerable increase in cost due to poor RF test yield.
Accordingly, it is the primary object of the present invention to provide power saturation control of Class C bipolar amplifiers.
This is achieved by connecting circuit means having a non-linear current voltage characteristic in series with the emitter RF choke of a common base bipolar transistor, which in turn increases the emitter bias voltage, and in turn forces the amplifier deeper into Class C operation. In a first aspect of the invention, a low ohmic resistor and a MOSFET device are connected in series with the emitter RF choke of a common base bipolar transistor Class C amplifier. An active feedback control loop implemented with conventional operational amplifier (OP AMP) circuitry controls the MOSFET device so as to operate in the knee region of its current-voltage characteristic during power overdrive to reduce the gain of the amplifier and thus maintain the power output substantially constant. In a second aspect of the invention, a non-linear circuit element connected in series with the emitter RF choke exhibits a low voltage drop if the supply current, resulting from a particular input signal, is below some critical level. At input levels forcing the current above the critical level, a rapid increase in emitter voltage occurs, limiting the gain by adjusting the conduction angle, effectively causing power saturation. A constant current diode having a high current, low knee voltage characteristic would provide such a non-linear characteristic and would be comprised of, for example, a depletion mode MOSFET or JFET where the gate is directly connected to the drain.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific examples, while disclosing the preferred embodiments of the invention, it is provided by way of illustration only, thus various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description.